The present invention relates to a process for the manufacture of fiber-reinforced articles, especially tubes, from chemically-modified protein as well as to tubes produced by the process and to the use thereof as synthetic sausage casings.
The term "fiber-reinforced tube of chemically modified protein" includes both tubes which, apart from the fiber insert in their wall, consist essentially of the chemically modified protein and also tubes which in addition contain small amounts of other chemical compounds, the presence of which plays a notable part in determining the properties of the tube. For example, there may also be present secondary chemical plasticising agents and/or water.
With the term "impregnated fibrous tube" is included a fibrous tube in which the fibers, especially paper fibers, are surrounded by the impregnating chemical substances, and in which essentially all of the cavities between the fibers forming the fibrous tube are filled by the chemical substance, and in which a layer of the chemical substance used for the impregnation is positioned on both surfaces.
Before the invention of synthetic polymers, e.g., polyamides, polyesters and polyurethanes, attempts were made to produce shaped bodies of synthetic materials from natural polymers, for example cellulose or protein. Some of these, such as, for example, cellulose hydrate films, have withstood the competition from synthetic polymers and are still produced in large quantities today. Shaped structures based on protein did not become of any importance, however, owing to the fact that the properties, especially the mechanical properties, of structures made from such materials are not so good. Of the shaped structures based on natural materials, a large share comprised synthetic sausage casings based on collagen. By dissolving collagen in alkali, spinning the solution into acid precipitating baths and subsequent hardening, tubes are produced with properties which are, however, by no means satisfactory. Even today shaped structures of casein are still produced according to a complicated and expensive process. For this purpose casein has a little water added to it and is plasticised at elevated temperatures under pressure. Subsequently the shaped article produced from the plasticised material is treated with a formaldehyde bath and thereby hardened. For this treatment, approximately one week per mm of wall thickness of the article is necessary.
Casein-based fibers are produced by dissolving casein in NaOH or KOH, spinning the solution into acid precipitation baths, and subsequently crosslinking the substance which forms the shaped structure by means of formaldehyde. These fibers could not, however, equal the mechanical properties of wool, which they were intended to replace, and therefore did not become of commercial importance.
The above survey shows that for the manufacture of tubes it has proved possible to successfully use only those natural polymers which themselves possess good film-forming properties, such as, for example, cellulose hydrate and collagen. According to the tests which have been published so far, natural products, such as casein, gelatins, amyloses, alginates or the like, always produce brittle articles which have too low a strength and insufficient elasticity.